A Farming cape can be purchased for 99,000 coins from Martin the Master Gardener at Draynor Village by players who have achieved level 99 Farming. It is the Cape of Accomplishment for the Farming skill. Farming capes are dark green in colour, and have a light green trim if the player has more than one level 99 skill.
This is a uncommon skillcape to see, because farming usually makes less money.
If you click on the farming icon on the skills tab, go to milestones, and click on the farming cape itself, you get the message "Unsurprisingly, capes don't need protection." This is the only known cape able to be clicked on.